1. Field of the Invention
The present invention relates to a multi-winding motor having N F-phase exciting windings, which are independent of each other.
2. Description of Related Art
As motors have been increasing in capacity, the driving devices for motors are also becoming larger to accommodate power elements and transformers of larger capacity and size, resulting in increased cost and leading to an impractical motor drive system.
As shown in Japanese Patent Application Laid-Open No. 1995-298685 (Patent Document 1), an attempt was made to reduce the size of the individual driving devices by connecting the driving devices (inverters or the like) separately to independent three-phase exciting windings of a multi-winding motor for synchronous driving.
An example of such arrangement is described below in detail with reference to FIG. 13. A multi-winding motor M has two three-phase exciting windings Sa and Sb, which are independent of each other. Separate driving devices, namely, inverters I1 and I2 are connected to the multi-winding motor M. These inverters I1 and I2 are synchronously driven to supply a drive current to the two three-phase exciting windings Sa and Sb. The inverters I1 and I2 are adapted to be supplied with power from a common DC power supply P. In the conventional multi-winding motors, however, impedance unbalance or fluctuation occurs among N F-phase exciting windings. In this specification, the wording “impedance unbalance” means that a large difference in impedance is generated among a plurality of exciting windings. When a motor is driven, a fluctuation is generated among the currents flowing through the N F-phase exciting windings, causing problems such as torque ripple, torque reduction, lowered efficiency, and deteriorated accuracy of positioning. Conventionally, in order to attain balanced current, an unbalance compensation voltage is applied to balance the current based on the error in current signals flowing through each of three phase exciting windings. However, the correction circuit has a limitation. Therefore, it is required to improve the impedance unbalance, which is the primary cause thereof.